This is a comprehensive review of K and W call letter assignments for AM band (mediumwave) radio stations in the United States, with an emphasis on stations that are on the "wrong side of the Mississippi".
Sections

The United States federal government began licencing radio stations in late 1912, and from the beginning it has assigned call letters starting with K and W to commercial and broadcasting stations. Moreover, from the start the policy has been that stations in the west normally got K-- calls, while W-- calls were issued to stations in the east. (Initially ship stations were the reverse, with W assignments in the west, and K in the east).

The original K/W boundary ran north from the Texas-New Mexico border, so at first stations along the Gulf of Mexico and northward were assigned W calls. It was only in late January, 1923 that the K/W boundary was shifted east to the current boundary of the Mississippi River. With this change, K's were assigned to most new stations west of the Mississippi, however, existing W stations located west of the Mississippi were allowed to keep their now non-standard calls.Click here for a detailed map showing the boundary change

NOTES: The source of the Mississippi River is in upper Minnesota, so using it as the K/W boundary leaves a gap in the northern part of the state. In 1987 the Federal Communications Commission noted that the current staff practice was to define the remainder of the boundary as "a line from the headwaters of the [Mississippi] to a point [at the Canadian border] just east of International Falls". This review generally omits stations in Louisiana and Minnesota, because the boundary has not been very strictly followed in those two states. Finally, this review lists stations according to their "community of licence", and does not include stations which only had transmitters on the other side of the divide. (Call letters are assigned according to the station's community of licence--the location of the station's transmitter, even if it is on the other side of the divide, does not matter).

Reviewing the stations on the AM band, many people have noticed that some of them have the "wrong" first letter for the side of the Mississippi River on which they are located. During the ninety years of call letter assignments for AM stations, I came up with six categories of non-conforming stations:

Stations located east of the Mississippi which were assigned calls from the KD-- ship block, instead of W--, during a June 1920 to April 1921 anomaly. (For some reason, during this anomaly almost all new land stations, east and west, got KU-- or KD-- four-letter calls. This included two broadcasting stations that just happened to be first licenced during this time: KDKA and KDPM).

Stations west of the Mississippi River that were licenced before the late January 1923 boundary shift, and were located in the slice of W territory that existed west of the Mississippi prior to the shift. (Originally about 170 stations, not including Minnesota and Louisiana. However, due to very high deletion rates plus later call changes, only eleven of these original calls survive: WEW, WHB, WKY, WOC, WOI, WBAP, WDAY, WJAG, WNAX, WOAI, and WTAW).

Portable stations (prior to 1928), which got W call letters because their original owners were located east of the Mississippi, but settled in a permanent home west of the Mississippi. (Four stations: WBBZ, WIBW, WLBN, and WMBH. There are no examples of a portable crossing in the other direction, i.e. no K portables "anchoring" in W territory).

Regular stations that changed their community of licence to the other side of the K/W divide. (Six stations: KFKX, KSGM, KWEM, WKBB, WPLX and KOTC. NOTE: This omits Louisiana and Minnesota.)

Assigned by the Government--three stations. KTGG in Spring Arbor (later Okemos), Michigan reportedly got a "K" callsign because someone at the FCC thought that the "MI" postal code stood for Missouri, a west-of-the-Mississippi state. Also, two additional call assignments appear to have been selected by government regulators: KYWA Chicago, a booster station for KYW, and KOP, licenced to the Detroit Police Department.

Finally, there are about a dozen stations for which I can not come up with any apparent reason--perhaps someone momentarily forgot about the policy, or where the boundary line was, or maybe I just need to do more research. The most prominent of these "undocumented" stations are KQV Pittsburgh, KSD Saint Louis (assigned before the boundary shift--now KTRS), and KYW Chicago (later Philadelphia and Cleveland).

27 Current AM-band K/W Boundary Exceptions (excluding LA & MN)

The above map lists the current exceptions to the standard of "K stations west of the Mississippi; W stations east" on the AM band. [27 Stations: 19 W's plus 8 K's. This map omits stations in Louisiana and Minnesota because the boundary has not been very strictly followed in those two states].

The color of the station's call letters reflects the reason for its exceptional status:

This map summarizes the current location of W and K stations on the AM band, by state. Yellow highlights the sixteen states mostly in the center of the country which currently have both K and W stations. In the west, in red, are the twelve states (including Hawaii), which have only had stations starting with K. Green designates the single state--Alaska--which had a W station in the past, but now has K stations exclusively. Light blue marks the two states--Mississippi and Ohio--which once had one or more K stations, but now exclusively have W stations. Finally, dark blue marks the nineteen eastern states which have only had W calls.

K/W Trivia

Boundary Switchover Date

Although I haven't been able to find out the exact date that the K/W boundary for land stations switched to the Mississippi River--the change was barely reported in the press--I have been able to narrow it down to late January, 1923. In reviewing the call letter assignments for broadcasting stations in the original W-west-of-the-Mississippi territory, it appears that the change took place sometime between January 19th and January 29th. The first date is when the State Normal School in Mayville, ND was assigned WRAC and the Taylor Radio Shop in Marion, KS was assigned WRAD. These apparently were the last two stations to fall under the old scheme. The next call assignment to a broadcast station located in the original W-west-of-the Mississippi territory that I am aware of did not come until ten days later. This station, licenced to Fred Mahaffey, Jr., in Houston, TX, was assigned KFCV, and from this point on K calls were issued to virtually all stations west of the Mississippi. (KFCV was deleted January 5, 1925).

Double Exceptions

Generally when one of the non-conforming stations switches to a new callsign, it also switches to the proper letter for its side of the dividing line. However, on a number of occasions the government has allowed stations to select new calls that continue to start with the "wrong letter": WHAA / WSUI Iowa City, IA; WKBB / WDBQ Dubuque, IA; WCK / WSBF and WEB / WIL / WRTH / WIL Saint Louis, MO; WJAD / WACO Waco, TX; WJAM / WMT Waterloo, IA; WOAW / WOW Omaha, NE; WNAD / WWLS Norman, OK; KEZK / KFNS Wood River, IL and KWEM / KWAM Memphis, TN.

U.S. Commonwealths, Territories and Possessions

Although Alaska and Hawaii were United States territories until 1959, for call letter purposes they were treated as western states, and normally received K callsigns. In addition, Puerto Rico, currently a U.S. commonwealth, and the Virgin Islands have been treated as eastern states, and both received a full range of W calls.

The Philippines, which were a United States possession until independence in 1946, were treated slightly differently. Stations here were only given calls starting with KZ--, until with independence the new nation was assigned its own block of international calls, and its broadcasting stations switched to calls starting with DX-- through DZ--.

The call letter policy for the remaining Pacific territories, in the years following World War II, has been a little more varied. Using the Mississippi River to divide the world into two halves, it appears that the entire Pacific Ocean should fall into K territory. (The International Dateline presumably doesn't have any significance for U.S. call letter policy. Its location is determined by the fact that it is halfway around the world from Greenwich in London, England, but because the K/W call letter boundary is the Mississippi River, not the Thames, it makes more sense to divide the world in halves at the 90 degree longitude mark, which is the location of the Mississippi River).

Most United States-affiliated broadcasting stations located in the Pacific have in fact been assigned K calls, although over the years there have also been a few W assignments. However, currently all the AM stations in Guam, American Samoa and the Commonwealth of the Northern Mariana Islands have K calls. As far as I can determine, all the broadcasting stations in Micronesia and the Marshall Islands were assigned W calls while they were U.S. territories, for reasons that are not readily apparent. However, after both these territories gained independence, the stations were assigned new calls from their own national allocations, starting with V6- for Micronesia, and V7- in the case of the Marshall Islands.

Extremes

Below are the most extreme "out of place" K's and W's on the AM band that I know of (locations are based on the station's community-of-licence, not their transmitter locations):

K's in the East (excluding Pacific)Farthest east: KYW-1060 Philadelphia (station moved from Chicago)

Below are the 52 AM stations I am aware of which have at times had both K and W calls. For each entry, I've tried to list all of the calls these stations have used over the years. If the final call includes a frequency, that means the station is still licenced, otherwise it has been deleted:

Listed below are the standard AM stations I was able to uncover which are exceptions to the current K/W dividing line of the Mississippi River.(This list only reviews regular broadcast service station authorizations. For call letter information for temporary grants, check out the information in United States Temporary Broadcast Station Grants: 1922-1928). Most of this information came from reviewing Commerce Department and Federal Radio Commission and Federal Communications Commission records, including the monthly Radio Service Bulletin. (For more information on government station lists, check out Early Radio Station Lists Issued By the U.S. Government)

As noted earlier, the reasons for the exceptions fall into six categories, plus "unknown":

1. June 1920-April, 1921 anomaly (KD-- calls for all)
2. West-of-the-Mississippi W territory (i.e. prior to the late-January, 1923 shift)
3. Portable station which a found permanent home on the opposite side of the Mississippi
4. Regular station that moved across the Mississippi
5. Call requested by owner
6. Government call assignment
?. Unknown reason

The "out of place" call letters, which are currently in use, are listed in bold, while stations that have been deleted or changed to "conforming" calls are in regular text. In the case where just a year or a month and year are listed, the date is the best estimate I can make using available records: